let draw =
  () {
    case Dom.Canvas.fromDomElement(canvas) {
      Maybe::Just(el) =>
        {
          el
          |> Canvas.setStrokeStyle(CanvasFillStyle::String("green"))
          |> Canvas.strokeRect(20, 10, 160, 100)
          ""
        }
      Maybe::Nothing =>
        ""
    }
  }
{
  await Timer.timeout(0)
  draw()
}
<canvas as canvas/>